Processes for the preparation of level-off DP cellulose have been disclosed, for example, in U.S. Pat. Nos. 2,978,446, 3,141,875 and 3,146,168. The term "level-off DP cellulose" which was coined by O.A. Battista in his essay "Hydrolysis and Crystallization of Cellulose, in Industrial and Engineering Chemistry", Vol. 42 (1950), 502-507, and which has generally been accepted in expert circles, comprises celluloses which have been subjected to partial hydrolysis under mild conditions and whose degree of polymerization, which correlates closely with the crystallite size of the starting celluloses, is termed LODP (levelling-off degree of polymerization) and is in a range of approximately 30-400.
The known processes for the preparation of level-off DP cellulose basically include partial hydrolysis of cellulose under conditions at which it is only the amorphous areas of these partially crystalline polysaccharides which are attacked, but these are dissolved completely. The partial hydrolysis is carried out, in these processes, in an acidic medium using aqueous solutions of sulphuric acid, FeCl.sub.3 and, preferably, of hydrochloric acid, the degree of polymerization during this treatment dropping down to the LODP, which, depending on the origin of the starting cellulose, can vary substantially. The resulting level-off DP cellulose is removed from the acid- or salt-containing mother liquor by means of filtration and subjected to laborious washing procedures to achieve an acid- or salt-free product involving a large amount of water and, if appropriate, using alkaline solutions, since the purity requirements stipulated for pharmaceutical purposes are very stringent. A resuspension of the filter cake is followed by spray-drying, which gives the fine, flowable end product.
An important problem in the conventional processes is the large amount of polluted waste water. Characteristic of an already optimized process are the following amounts, which, however, are often exceeded by a factor of 2-3 under current practical conditions:
______________________________________ Total amount of water 50 l/kg of level-off DP required cellulose NaCl obtained from the neu- 80 g/kg of level-off DP tralized acid cellulose Hemicellulose obtained 60 g/kg of level-off DP cellulose which corresponds to a COD of 64 g/kg of level-off DP cellulose ______________________________________